This was a little pet theory of mine that I'm glad to see actually turned out to be correct. See, one of the problems with having R2-D2 and C-3PO in the prequels is that their apparent lack of knowledge about what the hell is going on in the original trilogy never made any sense. How could they not know that Darth Vader is Anakin Skywalker? How could they not know who Obi-Wan Kenobi is? How could they not know that Luke and Leia are siblings? They were THERE!

Clearly the only possibility is that they've had their memories wiped by the start of Episode 4, but then I started thinking about it and I realized that it would be entirely possible that R2-D2 didn't have his mind wiped and he DID know everything that was going on in the original trilogy, he just didn't tell anyone because he's kind of a dick that way. If you ask me, one slick little addition they could've made to the Blu-Rays would be to have R2 make a kind of surprised or naughty whistle when Leia kisses Luke.